By popular demand, the archive of PC
Demos Explained is still available. Keep in mind it was last
updated in 1994.

Some general info

A demo is a non-interactive multimedia presentation made within
the computer subculture known as the demoscene.
Many demos are available online as well, with the popularity of online gaming increasing year after year. Checking out the top 10 online gambling sites allows you to make an informed decision about what to demo.
Demogroups create demos to demonstrate their
abilities in programming, music, drawing, and 3D modeling. The key
difference between a classical animation and a demo is that the
display of a demo is computed in real time, making computing power
considerations the biggest challenge.
Demos are mostly composed of
3D animations mixed with 2D effects and full screen effects.

The boot block demos of the 1980s, demos that were created to fit
within the small (generally 512 to 4096 bytes) first block of the
floppy disk that was to be loaded into RAM, were typically created
so that software crackers could boast of their accomplishment
prior to the loading of the game. What began as a type of
electronic graffiti on cracked software became an art form unto
itself. The demoscene both produced and inspired many techniques
used by video games and 3D rendering applications today - for
instance, light bloom, among others.

There are demos available for a great variety of computing
platforms. Currently, most new demos are native-code programs
designed to run on PCs under the Microsoft Windows operating
system, but demos are still actively being made for many other
machines including old and new computers, consoles and mobile
devices such as PDAs, mobile phones and pocket calculators.

The main historical platforms include Commodore 64, ZX Spectrum,
Atari ST and Commodore Amiga, and demo competitions for these
platforms are still relatively common on today's demo parties.
There are even demos running on such diverse platforms as VIC-20,
Commodore Plus/4, Atari 8-bit, Atari 2600, Amstrad CPC, Macintosh,
Game Boy, GP32 and PlayStation.